Saturday, November 8, 2014

The Deepest North Pacific Cyclone Ever?

After experiencing extratropical transition, former super typhoon Nuri has reached remarkable depths over the north Pacific.  The Ocean Prediction Center analysis of for 0600 UTC 8 November shows a central pressure of 924 mb.  To put that into perspective, the average sea level pressure is 1013 mb, so 924 mb means that there's about 9% less atmosphere over Nuri than one typically finds at sea level.

Source: Ocean Prediction Center
In all likelihood, Nuri is at or very near its maximum depth, so let's assume its minimum central pressure will be 924 mb.  In that instance, it won't be the deepest extratropical cyclone on record as some North Atlantic events that have gone lower than that, most recently in January 1993 when a sea level pressure of 913 mb was observed in the storm that caused the oil tanker Braer to break apart on the Shetland Islands (see Burt 2011).  

Determining whether or not Nuri is the deepest in the analyzed history of the North Pacific requires more sleuthing.  The analysis by Burt (2011) suggests that the previously deepest cyclone to affect Alaska reached 925 mb, so there's a chance Nuri tops this (barely!).  However, there are a number of issues that make determining if Nuri is record breaking for the north Pacific as a whole difficult:
  1. Cyclone central pressures derived from computer analyses over the oceanic regions prior to the satellite era and even since the advent of the satellite era have considerable uncertainty with regards to the minimum central pressure.  As a result, one can't simply do a search through digital analyses to see if Nuri is the deepest.
  2. Going through manual analyses is a tedious process.  Groups have done it, but often for subsets of years or decades and sometimes for regions rather than the entire north Pacific.  
  3. Manual analyses are not exact.  In the analysis above, the analyst has estimated the central pressure based on the surrounding observations (e.g., there is a 952 mb observation just east of the Kamchatka Peninsula), satellite imagery, and experience, but this is a source of uncertainty as well.  Ironically, pressure observations near the center of deep low centers are more rare today than in the past because ships and freighters typically know such storms are coming days in advance and adjust their routes to avoid mayhem. 
Based on the records I have access to, it appears Nuri is amongst the deepest of the deep.  Whether or not it ends up being the deepest on record depends ultimately on what the weather historians and analysts have to say.  

No comments:

Post a Comment